The present invention relates to a copier, printer, facsimile apparatus or similar image forming apparatus. More particularly, the present invention relates to a developing method capable of collecting a developer left on an image carrier after image transfer with a so-called cleanerless process and a developing device for practicing the same.
A developing system using a magnet brush is extensively applied to a photographic image forming apparatus of the type using a two-ingredient type developer, i.e., a toner and carrier mixture. In the magnet brush type developing system, a developer carrier conveys a developer deposited thereon in the form of a magnet brush and causes it to contact an image carrier, which carries a latent image thereon. An electric field is formed between the image carrier and a sleeve to which an electric bias is applied. The electric field causes the toner of the developer to selectively deposit on the latent image for thereby developing the latent image.
The developer carrier has a magnet roller accommodated in the sleeve, which is usually cylindrical. The magnet roller causes the developer deposited on the sleeve to rise in the form a magnet brush. The toner, which is charged to preselected polarity, deposits on the carrier present in the magnet brush. The magnet roller has a plurality of magnetic poles each being formed by a particular rod-like or similar magnet. Among the poles, a main pole is positioned on the surface of the sleeve in a developing region for causing the developer to rise. At least one of the sleeve and magnet roller moves relative to the other so as to cause the developer forming the magnet brush on the sleeve to move.
The developer brought to the developing region rises in the form of chains along magnetic lines of force issuing from the main pole of the magnet roller. The chains contact the surface of the image carrier while yielding. The chains feed the toner to the latent image while rubbing themselves against the latent image on the basis of a difference in linear velocity between the developer carrier and the image carrier. The developing region refers to a range over which the magnet brush on the developer carrier contacts the image carrier.
The image forming apparatus has customarily included a cleaner for collecting the toner left on the image carrier after the transfer of a toner image to a paper sheet or similar recording medium. Today, a cleanerless process is available that makes the cleaner unnecessary and thereby simplifies and miniaturizes the image forming apparatus.
Development using the toner and carrier mixture is superior to development using a single ingredient type developer, i.e., toner as to durability and reliability. However, development using the toner and carrier mixture needs a development gap as great as 500 xcexcm or above between the image carrier and the developer carrier. This gap is as small as several micrometers in development using only toner. The cleanerless process cannot surely collect the toner from the non-image area of the image carrier unless the electric field between the non-image area and the developer carrier is intensified. This kind of development therefore needs a great difference in potential and therefore high charge potential.
High charge potential, however, increases electrostatic stress to act on the image carrier to thereby reduce the life of the image carrier. Further, high charge potential aggravates the production of ozone and nitrogen oxides, resulting in an offensive smell and the blurring of an image. If the charge potential is lowered to solve the above problems, then the magnet brush strongly rubs itself against the image carrier. This causes the trailing edge of an image to be lost (omission of a trailing edge hereinafter) and obstructs the faithful reproduction of horizontal lines and dots. The resulting image is noticeably dependent on direction.
Technologies relating to the present invention are disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 7-84456, 8-160725, 9-236986, 2000-305360 and 2000-315001.
It is an object of the present invention to provide a method and an apparatus for development capable of insuring attractive images and surely collecting toner left on an image carrier with the cleanerless process.
It is another object of the present invention to provide an image forming apparatus including the above-described developing device.